In the manufacture of polymers by the bulk, or solution process it is generally required to significantly reduce the amount of residual monomer, and dimers, trimers and other oligomers and diluent, if present. This may be accomplished using a number of methods. For example the unfinished polymer may be passed through an extruder which has a section under vacuum. The polymer melt in the extruder is subject to working by the screw and a large surface area of melt is exposed to the vacuum. While the process is useful it subjects the polymer melt to shear which may cause polymer degradation. Additionally, it is usually carried out as a separate process step and may require additional manpower.
In bulk or solution polymerization where the polymer is reacted in the form of a melt, the melt may be heated then passed through a preheater into a devolatilization chamber. The melt also undergoes some shear as it is typically pumped from the reactor to the preheater using a gear pump. However, the shear is lower than that in an extruder. Typically the preheater is on top of the devolatilizer and is a tube and shell type heat exchanger. The polymer flows through the tubes and descends to the bottom of the devolatilizer as continuous strands. Unfortunately, in this process there are competing factors which affect the devolatilization. The residual monomers and/or dimers, and diluent, if present, will diffuse out of the melt faster at lower polymer melt viscosities. Unfortunately, at lower polymer melt viscosities, the polymer descends to the bottom of the devolatilizer faster and there is less time for the residual monomer, dimer, and diluent, if present, to diffuse out of the melt.
One method of overcoming this problem is to use a series of devolatilizers in sequence, as disclosed in U.S. Pat. No. 4,294,652 issued Oct. 13, 1981. While this approach may permit the polymer to be held in the form of a strand longer it represents an increase in capital cost, and maintenance costs.
Another approach is to flow the polymer melt down the surface of a plate heat exchanger as disclosed in U.S. Pat. No. 4,153,501 issued May 8, 1979. The present invention does not contemplate such a vertical plate heat exchanger.
A third approach is disclosed in applicant's U.S. Pat. No. 4,934,433 issued Jun. 19, 1990. The approach in that case is to use a horizontal polymer distributor which comprises a first enclosed section to permit polymer melt foaming or boiling and a second section to permit devolatilization and polymer melt strand formation. The apparatus disclosed in the U.S. patent application Ser. No. 271,636 does not contemplate the use of a weir. Furthermore, the patent application does not contemplate the use of a horizontal distributor tray as required in the present patent application.
Accordingly, the present invention provides a novel horizontal distributor means to be used in the upper end of a devolatilizer; and a process for its use. The polymer melts devolatilized using the distributor of the invention have a reduced content of residual monomer, dimer, and diluent, if present, relative to the use of a devolatilizer where such a horizontal distributor is not used.